OUTPUT PARAMETERS

DESCRIPTION

Note that use of this routine is deprecated as of MPI-2. Please use
MPI_Comm_create_keyval instead.
This deprecated routine is not available in C++.
Generates a new attribute key. Keys are locally unique in a process and
opaque to the user, though they are explicitly stored in integers. Once
allocated, the key value can be used to associate attributes and access
them on any locally defined communicator.
The copy_fn function is invoked when a communicator is duplicated by
MPI_COMM_DUP. copy_fn should be of type MPI_Copy_function, which is
defined as follows:
typedef int MPI_Copy_function(MPI_Comm oldcomm, int keyval,
void *extra_state, void *attribute_val_in,
void *attribute_val_out, int *flag)
A Fortran declaration for such a function is as follows:
SUBROUTINE COPY_FUNCTION(OLDCOMM, KEYVAL, EXTRA_STATE, ATTRIBUTE_VAL_IN,
ATTRIBUTE_VAL_OUT, FLAG, IERR)
1), the new attribute value is set to the value returned in
attribute_val_out. The function returns MPI_SUCCESS on success and an
error code on failure (in which case MPI_Comm_dup will fail).
copy_fn may be specified as MPI_NULL_COPY_FN or MPI_DUP_FN from either
C or Fortran; MPI_NULL_COPY_FN is a function that does nothing other
than return flag = 0, and MPI_SUCCESS. MPI_DUP_FN is a simple-minded
copy function that sets flag = 1, returns the value of attribute_val_in
in attribute_val_out, and returns MPI_SUCCESS.

NOTES

Key values are global (available for any and all communicators).
There are subtle differences between C and Fortran that require that
the copy_fn be written in the same language that MPI_Keyval_create is
called from. This should not be a problem for most users; only pro-
gramers using both Fortran and C in the same program need to be sure
that they follow this rule.
Even though both formal arguments attribute_val_in and
attribute_val_out are of type void*, their usage differs. The C copy
function is passed by MPI in attribute_val_in the value of the
attribute, and in attribute_val_out the address of the attribute, so as
to allow the function to return the (new) attribute value. The use of
type void* for both is to avoid messy type casts.
A valid copy function is one that completely duplicates the information
by making a full duplicate copy of the data structures implied by an
attribute; another might just make another reference to that data
structure, while using a reference-count mechanism. Other types of
attributes might not copy at all (they might be specific to oldcomm
only).
Analogous to copy_fn is a callback deletion function, defined as fol-
lows. The delete_fn function is invoked when a communicator is deleted
by MPI_Comm_free or when a call is made explicitly to MPI_Attr_delete.
delete_fn should be of type MPI_Delete_function, which is defined as
follows:
typedef int MPI_Delete_function(MPI_Comm comm, int keyval,
void *attribute_val, void *extra_state);
A Fortran declaration for such a function is as follows:
SUBROUTINE DELETE_FUNCTION(COMM, KEYVAL,ATTRIBUTE_VAL, EXTRA_STATE, IERR)
INTEGER COMM, KEYVAL, ATTRIBUTE_VAL, EXTRA_STATE, IERR
This function is called by MPI_Comm_free, MPI_Attr_delete, and
MPI_Attr_put to do whatever is needed to remove an attribute. The func-
tion returns MPI_SUCCESS on success and an error code on failure (in
which case MPI_COMM_FREE will fail).
delete_fn may be specified as MPI_NULL_DELETE_FN from either C or FOR-
TRAN; MPI_NULL_DELETE_FN is a function that does nothing, other than
returning MPI_SUCCESS.
The special key value MPI_KEYVAL_INVALID is never returned by MPI_Key-
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI:Exception object.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does
not guarantee that an MPI program can continue past an error.